Flight behaviours and energy savings in adult and juvenile house martins (Delichon urbicum) foraging near their breeding colony

Foraging is an extremely important behaviour for birds, especially during the breeding season, when they have to carry the cost of incubation and chick rearing, in addition to their own energy needs. Aerial insectivores perform most of their foraging behaviours in flight, so they have evolved various adaptations to reduce energy output whilst increasing energy input during this critical period. In this study, we recorded the 3D flight behaviours of 100 house martins (Delichon urbicum) flying near their colony during the breeding season in Rennes, France. We give a first description of the distribution of several kinematic and biomechanical variables (horizontal and vertical speed, rates of change in kinetic and potential energy, turning radius of curvature and centripetal force), compare flapping and gliding flight, and describe several strategies used by flying house martins to save energy, such as environmental energy extraction (thermal soaring) and optimisation of flight speed according to wind speed and direction. We also report an effect of temperature, solar radiation and humidity on the mean vertical speed of gliding birds, highlighting the effect of weather on the availability of external energy sources such as thermal updrafts. Finally, we compare the distribution of flight speed and vertical speed between 5 juveniles identified using magnified photographs and 20 adults recorded during the same field sessions, and we show that during flapping flight, juveniles exhibit higher, more variable airspeed than adults, suggesting that their flight behaviours are not immediately fine-tuned after leaving the nest.Significance statementAerial insectivores use various strategies to reduce the cost of foraging flight. Using an optical tracking method, we recorded the 3D flight behaviours of house martins (Delichon urbicum) flying near their colony during the breeding season. We describe the distribution of several biomechanical variables and show that house martins use external energy sources such as thermal updrafts and also adapt their airspeed to wind speed and direction, supporting the predictions on optimal cost of transport in birds. Moreover, juveniles were also recorded, and they show a greater variability in flight speed, possibly because they may not be as accurate as adults in finely adjusting their speed and altitude. Our findings add to the existing literature showing energy-saving strategies in aerial insectivores, and also study an ontogenetical aspect rarely explored.

Automated summary

In this study, the flight behaviors of house martins during the breeding season were recorded, and several kinematic and biomechanical variables were described. The house martins were found to use various strategies to save energy, such as environmental energy extraction and optimization of flight speed according to wind conditions. The study also identified the impact of temperature, solar radiation, and humidity on gliding birds' vertical speed and compared the flight behaviors of juvenile and adult birds.